This invention generally relates to metallic alloy compositions. More specifically, the invention relates to nickel/cobalt alloys useful for high temperature applications, and related articles.
Superalloys are often the materials of choice for components intended for high-temperature environments. (The term “superalloy” is usually intended to embrace complex cobalt- or nickel-based alloys which include one or more other elements such as aluminum (Al) and chromium (Cr)). As an example, turbine blades and other parts of turbine engines are often formed of nickel-based superalloys because they need to maintain their integrity at temperatures of at least about 1000-1150° C. The alloys can be formed by a variety of processes, such as conventional casting, unidirectional casting, and single crystal techniques. A number of treatment steps usually follow casting, such as “solid-solutioning”, aging treatments, and precipitation-strengthening steps. The alloys may also be provided with an environmental protection coating.
The addition of various elements to the nickel (Ni) matrix results in the formation of the “L12”-structured phase, via a precipitation mechanism. As those in the art understand, the presence of the L12 phase provides greater strength to the alloy, at very high use temperatures. In fact, in many instances, the L12 phase exhibits an inverse temperature dependence, in which strength becomes higher with rising temperature.
The cobalt-based alloys are also of special interest for certain end uses. As an example, these alloys sometimes exhibit higher melting temperatures than their nickel counterparts. Depending on specific formulations, the cobalt (Co) alloys can also sometimes provide enhanced corrosion resistance in a variety of high-temperature environments which contain corrosive gases.
Up until recently, cobalt-based alloys which also include the desirable L12 phase appeared to be unavailable. However, in U.S. Patent Publication 2008/0185078, Ishida et al describe cobalt-based alloys with high heat resistance and strength, and which contain a precipitated L12 phase. The L12 phase in this instance is an intermetallic compound of the formula Co3(Al, W). While the alloy compositions in Ishida may contain a number of other elements, most of the compositions appear to be based on relatively large amounts of cobalt, aluminum, and tungsten.
Metallurgists understand that nickel and cobalt alloys used in demanding applications often require a very careful balance of properties. Just a few of these properties are mentioned here: strength (at high and medium temperatures), oxidation resistance, ductility, and corrosion resistance. Other properties and characteristics include “castability”, weight, and cost. In highly demanding service environments, achieving a necessary balance between all of these properties represents an ever-increasing challenge to the alloy formulator.
Furthermore, manufacturing flexibility in preparing a desired alloy has become an important consideration in a commercial setting—especially in an era of high energy costs and raw material costs. While small alloy samples in the laboratory can be formulated and cast very precisely, that type of precision is often not attainable in a large, commercial foundry-type operation, where alloy melts and billets can weigh up to 20,000 pounds. If the cast alloy is found to be “off-spec” and inferior, e.g., due to a formulation error, it may have to be re-melted or scrapped. Either result can represent a serious production problem. Therefore, alloy formulations in which the levels of certain constituents can be changed to some degree, without adversely affecting the properties of the final casting, would be of considerable value in an industrial setting.
With these considerations in mind, new superalloy compositions would be welcome in the art. The alloys should exhibit a desirable combination of the properties noted above, such as environmental resistance and high-temperature strength. They should also exhibit good “manufacturability” characteristics, which can provide important commercial advantages in the industrial environment.